Explain the functions of growth hormone and thyroxine (also \(\mathrm{T}_{3}\) ) as they are related to normal growth. (pp. 234 , 236)

Growth hormone, also known as somatotropin, is secreted by the anterior pituitary gland. Its primary function is to stimulate the growth of various tissues in the body, which includes the growth of bones, muscles, and organs. Growth hormone acts directly on cells and indirectly through the production of another hormone known as Insulin-like Growth Factor 1 (IGF-1). Some of the specific functions of growth hormone related to normal growth include: 1. Increased cell growth and cell division, resulting in an increase in skeletal length and muscle mass. 2. Promotion of protein synthesis, thereby helping build the structural and functional components of cells. 3. Regulation of metabolism, leading to the mobilization and utilization of fat stores in the body. 4. Cooperation with other hormones, such as thyroxine, to produce synergistic effects on growth.

Thyroxine (T4) and triiodothyronine (T3) are thyroid hormones produced by the thyroid gland. Though thyroxine is more abundant, T3 is more potent and is responsible for the majority of the biological effects of thyroid hormones. Thyroid hormones play a crucial role in maintaining normal growth and development by influencing various metabolic processes in the body. Some of the significant functions of thyroxine related to normal growth include: 1. Regulation of basal metabolic rate (BMR), which determines the energy consumption, heat production, and indirectly, the growth rate. 2. Enhancement of protein synthesis, contributing to tissue growth and maturation. 3. Stimulation of the synthesis and breakdown of carbohydrates and fats for energy generation, which is necessary for proper growth. 4. Synergism with growth hormone to improve the responsiveness of tissues to growth stimuli. 5. Regulation of brain development and maturation, particularly during the fetal stage and infancy.

Growth hormone and thyroxine work together synergistically to promote normal growth and development. Here's how they interact: 1. Growth hormone stimulates the production of IGF-1, which in turn, increases the production of T3 in the liver. 2. Thyroid hormones (especially T3) enhance the responsiveness of target tissues to growth hormone and IGF-1 by influencing the expression of growth hormone receptors. 3. Growth hormone and thyroxine together influence protein synthesis, energy metabolism, and cell proliferation and differentiation, ultimately leading to coordinated growth of the organism. 4. A deficiency or excess of either hormone can cause growth abnormalities - growth hormone deficiency can result in short stature, while thyroxine deficiency can lead to growth retardation and mental developmental issues. In conclusion, growth hormone and thyroxine play essential roles in promoting normal growth and development. They work in coordination by influencing various metabolic processes, and their balanced secretion is crucial for proper growth and functioning of the body.